I use them, because the sticks on the PS3 controller are awful. They're great though for both fixing the issue of them being outwardly concave and for allowing better accuracy due to the extended length of the stick (which, in turn, allows you to play on a higher sensitivity). I've had them for a few years, so they're the original ones. I was relieved to see that the PS4 controller's sticks seem to be inwardly concave similar to the 360s controllers.
I use the Kontrol Freek Havoc's. I only use one and it's on R3. I tried them on both L3 and R3 but the L3 was just to hard to get used to. I really haven't noticed any extra accuracy as some have suggested. But I have noticed a significant reduction in hand fatigue in my right hand when playing for long periods of time. Also, because the Kontrol Freek extends the R3 button/joystick it provides a greater amount of leverage making it a lot easier to drop to prone when your controller is set on tactical. Overall, I think it was worth the money.
I Like them and I only have one on the R3 but for $12 bucks it was worth it, I have seen accuracy increse slightly.
Moved to OffTopic.
I use the KF CQC (the ones with the shorter sticks). I like them a lot. I'm a rusher and I can wear my thumb out on the concave tops of the regular controller sticks.
I am a rusher! I would truly like to test how much rushing you do, I am a counter rusher! buuuuhhhaaa.